U.S. Pat. No. 8,211,197, the entirety of which is incorporated-by-reference, describes an air cleaner filter assembly. The filter assembly includes a primary filter, a secondary filter set apart from the primary filter, and a frame fixed to and configured to support the main filter and the auxiliary filter. The frame is fittable in and removable from a housing of the air cleaner, and comprises a sealing element passing between the primary filter and the secondary filter.
During normal operation, air passes through only the primary filter and the secondary filter is sealed from the primary filter by the sealing element. When the primary filter is blocked by heavy dirt, dust, or snow, the secondary filter receives air flow and provides clean air to the vehicle engine. However in extreme conditions such as heavy mud, it is possible for pressure to cause the primary filter to collapse preventing air flow from the secondary filter.